Prime Minister Stephen Harper is seeking to overhaul his office amid a damaging controversy that has left him facing pressure to regain control of the national political agenda.

The departure of director of communications Andrew MacDougall – the seventh in seven years – and the expected appointment of a longtime Harper loyalist are the latest elements of a shakeup in the Prime Minister’s Office that comes as Mr. Harper heads into the second half of his term. The Prime Minister, who shuffled his cabinet in July, has been weighed down by a controversy over Senate spending that has claimed his chief of staff, Nigel Wright, and by a struggle throughout the spring to maintain control of his base.

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The Prime Minister’s new chief of staff, Ray Novak, has been looking to recover momentum following the controversial payment of $90,000 by Mr. Wright to Senator Mike Duffy, and now faces a renewed onslaught of questions stemming from the audit report on Senator Pamela Wallin’s spending.

The revamped PMO under Mr. Novak is expected to seek greater discipline from its members with the upcoming arrival of Jenni Byrne, a Harper loyalist known for a no-holds-barred approach to politics who is slated to become a deputy chief of staff.

“This is Ray Novak putting his stamp on the PMO … shaking up the PMO two years out of the next election,” said a Conservative insider, who spoke on condition of anonymity.

Mr. Harper’s top Quebec adviser, former MP André Bachand, has already been replaced by Catherine Loubier, a former Conservative staffer. And one of the people in the PMO who knew of the $90,000 payment to Mr. Duffy, Chris Woodcock, left his position as director of issues management last month.

Mr. Novak, who is Mr. Harper’s longest-serving aide, is looking to staff the PMO with “workhorses” who will aim to deliver a series of political victories for the government in the fall, sources said.

The next steps for the government include a Speech from the Throne, likely in October, followed by the Conservative Party convention in Calgary, which was delayed by the flooding in June. The party will also have to fight four by-elections this year, two of them on supposedly safe terrain in Manitoba.

However, the opposition parties are looking to exploit any anger over recent controversies – from the Senate scandal to the backbench revolt over abortion – within the Conservative base. The NDP and the Liberal Party of Canada have hit hard at the Conservatives in the House of Commons and across the country in recent months. This has left Mr. Harper struggling to communicate his message to Canadians, fuelling his thirst for a fresh start in the fall and the need for the ongoing changes inside his government.

“This is about the government regaining control of the agenda,” the insider said.

Mr. MacDougall will stay in his job in Ottawa until early September, allowing him to attend the upcoming G20 summit in St. Petersburg. He will then join the U.K. offices of MSLGROUP, which describes itself as Publicis Groupe’s strategic communications and engagement network.

In a goodbye note to his colleagues, Mr. MacDougall said his desire to move to London is “long-standing and deep-seated.” He thanked his former bosses and colleagues, including Mr. Harper, whom he has served since 2008.

“Most of all, I am grateful for the Prime Minister's confidence. It has been a rare privilege to watch firsthand how the Prime Minister has led Canada through these tough economic times. To have been a small part of this endeavour will forever rank as one of my proudest accomplishments,” he said.

Mr. MacDougall stepped into his current position in 2012 after his predecessor, Angelo Persichilli, said he couldn’t handle the stress of the job.

In a statement, Jeremy Sice, CEO of MSLGROUP in the U.K., said Mr. MacDougall brings much experience gained in his job in the centre of power in Ottawa.